The present invention relates to a pressure differential indicating apparatus and a measuring piston of a hydraulic bridge. More particularly, it relates to a throughflow quantity measuring apparatus having a measuring chamber positioned between a test object which emits the throughflow quantity to be measured to said chamber and therefrom to a throughflow quantity valve element, said measuring chamber being associated with a pressure differential sensor having a movable wall which controls the volume of the measuring chamber and is acted upon by the pressure of a measuring medium that pulsates at slightly different pressures, with said pressure differential sensor being provided with a signal transmitter for controlling the measuring chamber volume.
Hyraulic measuring bridges are designed to function on a comparison principle, wherein the throughflow quantity emitted by a test object is compared with the throughflow quantity emitted by a throughflow quantity valve element which serves as the standard. When this type of measuring bridge is used in a throughflow quantity measuring apparatus, the calibrated throughflow quantity valve element is readjusted until such time as the volume of the measuring chamber remains constant, that is until an equal throughflow quantity is conveyed by the throughflow quantity valve element and by the test object. When this hydraulic measuring bridge is used with a throughflow quantity measuring apparatus, the operation of the apparatus is reversed, that is, the throughflow quantity valve element is set to a fixed predetermined value and the test object is thereafter adjusted either by hand or automatically until such time as its throughflow quantity corresponds to the throughflow quantity of the throughflow quantity valve element and that point is reached when the measuring chamber volume remains constant. The precision and functional capability of such measuring instruments depends entirely on the precision and functional capability of the pressure differential sensor attached to the measuring chamber; thus the potentially present friction enters directly into the measured result and leakage losses likewise falsify the result.
A pressure differential indicating apparatus is known which features a so-called swimming measuring piston which acts as the movable wall and is of a type which is fitted into a measuring cylinder with a clearance of a few thousanths of a millimeter with the piston being set into motion by the measuring medium and without having any additional sealing means serves to indicate the volume changes or pressure changes through its optically measured position changes. Such a measuring piston has the disadvantage of reacting very sensitively to entrained particulate matter within the measuring medium and a direct articulation of a non-contacting electrical excursion sensor is thus not possible, so that the deployment of the appertaining pressure differential indicating apparatus remains largely restricted to laboratory operations and its usage in a mass production environment for throughflow quantity measuring or for throughflow quantity setting is as a result extremely difficult.
Also well-known in the prior art is a pressure differential indicating apparatus of a hydraulic measuring bridge of the type referred to above, wherein a throughflow control valve provided with two diaphragms serves as the pressure differential sensor and this throughflow control valve regulates the fluid stream for the propulsion of a hydraulic motor which drives a pump that serves as the throughflow quantity valve element. The revolutions per unit time of the hydraulic motor in such a device are readjusted until such time as the conveyance quantity of the hydraulic pump that serves as the throughflow quantity valve element equals the conveyance quantity of the test object. In this exclusively hydraulic-mechanical apparatus, the double diaphragm works relatively satisfactorily only near its null position. However, during extreme deflections an amplified friction appears, the magnitude of which can become uncontrollably high and can thus not be compensated for by a corresponding calibration of the instrument. Such an apparatus cannot follow rapid and larger volume changes, and is therefore not suitable in most cases of application.